[tel-00726959, v1] Caractériser le milieu interstellaire ... - HAL - INRIA

J. R. Goicoechea et al.: The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds 773tel-00726959, version 1 - 31 Aug 2012Fig. 1. DCO + J = 3–2 and 2–1 (IRAM-30 m; from Pety et al. 2007), H 13 CO + J = 1–0 (PdBI) and 3–2 (IRAM-30 m) line integrated intensitymaps, aromatic infrared band emission (ISOCAM, from Abergel et al. 2003) and HCO (PdBI, from Gerin et al. 2009). Maps have been rotatedby 14 ◦ counter–clockwise around the projection center, located at (δx,δy) = (20 ′′ , 0 ′′ ), to bring the illuminated star direction in the horizontaldirection. The horizontal zero has been set at the cloud edge (δx = 0 ′′ ). The H 13 CO + ,DCO + and HCO emission is integrated between 10.1 and11.1 km s −1 . Integrated intensities are expressed in the T mb scale. Contour levels are displayed on the grey scale lookup tables. The red verticalline shows the PDR edge and the green crosses shows two representative positions: the “shielded core” (the DCO + emission peak at δx ∼ 45 ′′ ;Pety et al. 2007) andthe“PDR” (the HCO emission peak at δx ∼ 15 ′′ ; Gerin et al. 2009). The dashed blue line shows the horizontal cut analyzedin this work.distribution and it mostly delineates the dense core that coincideswith the DCO + emission peak. Nevertheless, while DCO +is not detected in the illuminated edge, H 13 CO + does show afaint emission in the PDR. Therefore, the small field-of-viewshowninFig.1 contains two different environments: a warmPDR and a cold core shielded from the external UV radiationfield. In the following sections we analyze these emission mapsto determine the ionization fraction gradient in the region.Figure 2 shows long integration spectra of the HOC + ,H 13 CO + and HCO + J = 1–0 lines towards the PDR. Thisis the first detection of the HOC + reactive ion towards theHorsehead, and adds to previous detections in interstellar environmentswith high electron abundances (Woods et al. 1983;Ziurys & Apponi 1995; Fuente et al. 2003; Rizzo et al. 2003;Savage & Ziurys 2004; Liszt et al. 2004). H 12 CO + lines are opticallythick, as shown by the low H 12 CO + /H 13 CO + J = 1–0 lineintensity ratio (∼7), much lower than the expected 12 C/ 13 C ≃60 abundance ratio (Langer & Penzias 1990; Savage et al. 2002)and references therein). The high opacity of H 12 CO + lines eventowards the PDR justifies the use of H 13 CO + lines as tracers ofthe HCO + abundance.Table 3. Main spectroscopic parameters of the studied lines.Species Transition Frequency A ij E uppJ upp −J low (GHz) (s −1 ) (K)HCO + 1–0 89.188523 4.2 × 10 −5 4.3HOC + 1–0 89.487414 2.2 × 10 −5 4.3H 13 CO + 1–0 86.754288 3.9 × 10 −5 4.2CO + 2(5/2)–1(3/2) 236.062578 4.7 × 10 −4 17.2H 13 CO + 3–2 260.255339 1.3 × 10 −3 25.0DCO + 2–1 144.077289 2.1 × 10 −4 10.4DCO + 3–2 216.112582 7.7 × 10 −4 20.73. Analysis: modelsIn this work we couple the depth-dependent abundances predictedby a PDR model (for the varying physical conditions prevailingin the Horsehead edge) with detailed excitation and radiativetransfer calculations adapted to the cloud geometry. Thistechnique allows us to analyze different chemical models by directcomparison with observed line intensities. This methodologywas introduced to study our interferometric CS and C 18 O